This invention relates to radioactivity well logging and more particularly to methods and systems useful in well logging employing penetrating radiation at a plurality of discrete energy levels.
In the petroleum industry, certain well logging practices involve subjecting a subterranean medium composed of several substances to radiation and measuring one or more effects of such radiation to determine the identity of substances in the system or the relative proportions of such substances. For example, in the investigation of subterranean earth formations, various radioactivity logging techniques may be employed in order to characterize such formations with regard to their fluid or mineral content, matrix lithology, density, porosity, or to provide for stratigraphic correlation. In these techniques, the formation under investigation is irradiated with a steady state or pulsed primary radiation source and the resulting radioactive effect is measured in order to characterize the formation with respect to one or more of the parameters noted above. For example, the formation may be irradiated with repetitive bursts of fast neutrons, normally neutrons exhibiting an energy greater than one Mev. When the fast neutrons enter the formation, they are moderated to lower energy levels by the nuclear collision processes of elastic and inelastic scattering. As the neutrons are moderated or slowed down, they reach the epithermal range and thence are further moderated until they reach the thermal neutron range. Once a neutron reaches the thermal energy level, it diffuses through the formation until it is captured by a nucleus with the resultant emission of one or more gamma rays. The populations of neutrons at the various energy levels decay with time following the primary irradiation and thus offer means of characterizing the formation. For example, the rate of decay of epithermal neutrons may be used to give a quantitative measure of hydrogenous material present in the formation which in turn may be indicative of the formation porosity. The rate of decay of thermal neutrons within the formation may be used to characterize the formation as to its chlorine, and thus salt water content. The thermal neutron decay rate can be determined by successive measurements of thermal neutrons or of capture gamma rays.
Various radioactivity logging processes are also carried out employing steady state radiation source. For example, in neutron porosity logging, a steady state neutron source is employed to irradiate the formation under investigation with fast neutrons. The porosity of the formation may then be determined by measuring thermal neutrons with two detectors at different spacings from the source or by measuring epithermal neutrons with a single detector.
Another widely used radioactive logging technique is the gamma ray density procedure. In this procedure, the absorption of gamma rays within the formation is used as an indication of the density and hence the porosity of the formation. In gamma ray density logging, the formation under investigation is irradiated with gamma rays which are subject to attenuation by one or more of several mechanisms. A portion of the gamma rays is scattered from the formation back into the bore hole where they are detected. The intensity of the gamma rays detected is a function of the electron density of the formation which provides a close approximation of the actual bulk density. As disclosed in U.S. Pat. No. 3,202,822 to Kehler, two detectors spaced from a gamma ray source by unequal distances may be employed in order to minimize bore hole effects such as may be due to the bore hole fluid, irregularities in the wall of the well, or mud cake, casing, cement, etc. in the well. In addition to employing a pair of detectors, Kehler also discloses the use of a collimated source and collimated detectors in order to reduce variables associated with the geometry of the detected radiation and the different energies of Compton scattered photons from different scattering angles.